U.S. patent application number 15/651253 was filed with the patent office on 2017-11-02 for laser apparatus and extreme ultraviolet light generation system.
This patent application is currently assigned to GIGAPHOTON INC.. The applicant listed for this patent is GIGAPHOTON INC.. Invention is credited to Hideo HOSHINO, Yoshiaki KUROSAWA.
Application Number | 20170317464 15/651253 |
Document ID | / |
Family ID | 56878654 |
Filed Date | 2017-11-02 |
United States Patent
Application |
20170317464 |
Kind Code |
A1 |
KUROSAWA; Yoshiaki ; et
al. |
November 2, 2017 |
LASER APPARATUS AND EXTREME ULTRAVIOLET LIGHT GENERATION SYSTEM
Abstract
A laser apparatus may include a master oscillator, a plurality
of amplifiers, a photodetector device configured to detect a light
beam traveling back along a laser beam path, and a controller. The
photodetector device may include a first photodetector configured
to detect energy of a light beam traveling back along the laser
beam path and a second photodetector configured to detect power of
the light beam traveling back along the laser beam path. The
controller may be configured to determine that a return beam is
generated when the intensity of the energy detection signal exceeds
a first threshold. The controller may be configured to determine
that a self-oscillation beam is generated when the intensity of the
power detection signal exceeds a second threshold.
Inventors: |
KUROSAWA; Yoshiaki;
(Oyama-shi, JP) ; HOSHINO; Hideo; (Oyama-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GIGAPHOTON INC. |
Tochigi |
|
JP |
|
|
Assignee: |
GIGAPHOTON INC.
Tochigi
JP
|
Family ID: |
56878654 |
Appl. No.: |
15/651253 |
Filed: |
July 17, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2015/056653 |
Mar 6, 2015 |
|
|
|
15651253 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 3/10069 20130101;
H01S 3/095 20130101; H01S 2301/02 20130101; H01S 3/09702 20130101;
H01S 3/034 20130101; H01S 3/1003 20130101; H01S 3/2316 20130101;
H01S 3/2308 20130101; H01S 3/0064 20130101; H01S 3/0071 20130101;
H01S 3/08072 20130101; H05G 2/008 20130101 |
International
Class: |
H01S 3/10 20060101
H01S003/10; H01S 3/034 20060101 H01S003/034; H01S 3/00 20060101
H01S003/00; H01S 3/08 20060101 H01S003/08; H01S 3/095 20060101
H01S003/095; H01S 3/23 20060101 H01S003/23 |
Claims
1. A laser apparatus comprising: a master oscillator configured to
output a laser beam to travel along a laser beam path; a plurality
of amplifiers configured to amplify the laser beam outputted by the
master oscillator on the laser beam path; a photodetector including
a first photodetector configured to detect energy of a light beam
traveling back along the laser beam path and a second photodetector
configured to detect power of the light beam traveling back along
the laser beam path; and a controller configured to receive an
energy detection signal from the first photodetector and a power
detection signal from the second photodetector, monitor intensities
of the energy detection signal and the power detection signal,
determine that a return beam is generated when the intensity of the
energy detection signal exceeds a first threshold, determine that a
self-oscillation beam is generated when the intensity of the power
detection signal exceeds a second threshold, and store
determination results about the return beam and the
self-oscillation beam to a memory.
2. The laser apparatus according to claim 1, further comprising a
display device, wherein the controller is configured to output the
determination results to the display device.
3. The laser apparatus according to claim 1, wherein the controller
is configured to: keep operation of the plurality of amplifiers and
stop the master oscillator in a case of determining that a return
beam is generated; and stop the master oscillator and the plurality
of amplifiers in a case of determining that a self-oscillation beam
is generated.
4. The laser apparatus according to claim 3, wherein the controller
is configured to resume operation of the master oscillator and
adjust a focus of a pulse laser beam with respect to a target in a
case of determining that a return beam is generated.
5. An extreme ultraviolet light generation system configured to
generate extreme ultraviolet light by irradiating a target with a
pulse laser beam to generate plasma, the extreme ultraviolet light
generation system comprising: the laser apparatus according to
claim 1 configured to output the pulse laser beam; a chamber; a
target supply unit configured to supply a target into the chamber;
and a laser beam focusing optical system configured to focus the
pulse laser beam outputted by the laser apparatus onto the
target.
6. A laser apparatus comprising: a master oscillator configured to
output a laser beam to travel along a laser beam path; a plurality
of amplifiers configured to amplify the laser beam outputted by the
master oscillator on the laser beam path; a photodetector device
configured to detect a light beam traveling back along the laser
beam path; and a controller configured to determine that a return
beam is generated when intensity of a detection signal from the
photodetector device has been higher than a first threshold for a
time longer than 0 and equal to or shorter than a second threshold,
determine that a self-oscillation beam is generated when intensity
of the detection signal has been higher than a third threshold for
a time longer than a fourth threshold which is equal to or longer
than the second threshold, and store determination results about
the return beam and the self-oscillation beam to a memory.
7. The laser apparatus according to claim 6, wherein the
photodetector device includes a single photodetector, and wherein
the first threshold and the third threshold are the same value and
the second threshold and the fourth threshold are the same
value.
8. An extreme ultraviolet light generation system configured to
generate extreme ultraviolet light by irradiating a target with a
pulse laser beam to generate plasma, the extreme ultraviolet light
generation system comprising: the laser apparatus according to
claim 6 configured to output the pulse laser beam; a chamber; a
target supply unit configured to supply a target into the chamber;
and a laser beam focusing optical system configured to focus the
pulse laser beam outputted by the laser apparatus onto the target.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation application of
International Application No. PCT/JP2015/056653 filed on Mar. 6,
2015, the content of which is hereby incorporated by reference into
this application.
BACKGROUND
1. Technical Field
[0002] This disclosure relates to a laser apparatus and an extreme
ultraviolet light generation system.
2. Related Art
[0003] In recent years, semiconductor production processes have
become capable of producing semiconductor devices with increasingly
fine feature sizes, as photolithography has been making rapid
progress toward finer fabrication. In the next generation of
semiconductor production processes, microfabrication with feature
sizes at 70 nm to 45 nm, and further, microfabrication with feature
sizes of 32 nm or less will be required. In order to meet the
demand for microfabrication with feature sizes of 32 nm or less,
for example, the development of an exposure apparatus is expected
in which a system for generating extreme ultraviolet (EUV) light at
a wavelength of approximately 13 nm is combined with a reduced
projection reflective optical system.
[0004] Three kinds of systems for generating EUV light are known in
general, which include a Laser Produced Plasma (LPP) type system in
which plasma is generated by irradiating a target material with a
laser beam, a Discharge Produced Plasma (DPP) type system in which
plasma is generated by electric discharge, and a Synchrotron
Radiation (SR) type system in which orbital radiation is used to
generate plasma.
SUMMARY
[0005] An example of the disclosure may be a laser apparatus
including a master oscillator, a plurality of amplifiers, a
photodetector device, and a controller. The master oscillator may
be configured to output a laser beam to travel along a laser beam
path. The plurality of amplifiers may be configured to amplify the
laser beam outputted by the master oscillator on the laser beam
path. The photodetector device may include a first photodetector
configured to detect energy of a light beam traveling back along
the laser beam path and a second photodetector configured to detect
power of the light beam traveling back along the laser beam path.
The controller may be configured to receive an energy detection
signal from the first photodetector and a power detection signal
from the second photodetector, monitor intensities of the energy
detection signal and the power detection signal, determine that a
return beam is generated when the intensity of the energy detection
signal exceeds a first threshold, determine that a self-oscillation
beam is generated when the intensity of the power detection signal
exceeds a second threshold, and store determination results about
the return beam and the self-oscillation beam to a memory.
[0006] Another example of the disclosure may be a laser apparatus
including a master oscillator, a plurality of amplifiers, a
photodetector device, and a controller. The master oscillator may
be configured to output a laser beam to travel along a laser beam
path. The plurality of amplifiers may be configured to amplify the
laser beam outputted by the master oscillator on the laser beam
path. The photodetector device may be configured to detect a light
beam traveling back along the laser beam path. The controller may
be configured to determine that a return beam is generated when
intensity of a detection signal from the photodetector device has
been higher than a first threshold for a time longer than 0 and
equal to or shorter than a second threshold, determine that a
self-oscillation beam is generated when intensity of the detection
signal has been higher than a third threshold for a time longer
than a fourth threshold which is equal to or longer than the second
threshold, and store determination results about the return beam
and the self-oscillation beam to a memory.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Hereinafter, selected embodiments of the present disclosure
will be described with reference to the accompanying drawings.
[0008] FIG. 1 schematically illustrates an exemplary configuration
of an LPP type EUV light generation system.
[0009] FIG. 2 illustrates details of a configuration example of an
EUV light generation system.
[0010] FIG. 3 illustrates the definition of divergence angle.
[0011] FIG. 4A illustrates a state of a return beam in the case
where the focal point of the pulse laser beam is set to the target
position.
[0012] FIG. 4B illustrates a state of a return beam in the case
where the focal point of the pulse laser beam is set to a position
upstream of the target position.
[0013] FIG. 4C illustrates a state of a return beam in the case
where the focal point of the pulse laser beam is set to a position
downstream of the target position.
[0014] FIG. 5A schematically illustrates a temporal waveform of a
return beam.
[0015] FIG. 5B schematically illustrates a temporal waveform of a
self-oscillation beam.
[0016] FIG. 6 illustrates a configuration example of an EUV light
generation system in Embodiment 1.
[0017] FIG. 7 is a graph for illustrating an example of a way for a
laser controller to determine whether a return beam or
self-oscillation beam is present in Embodiment 1.
[0018] FIG. 8 is a flowchart of operation of the laser controller
to detect and address a return beam or self-oscillation beam in
Embodiment 1.
[0019] FIG. 9 is a detailed flowchart of focus adjustment in
Embodiment 1.
[0020] FIG. 10 illustrates a configuration example of an EUV light
generation system in Embodiment 2.
[0021] FIG. 11A illustrates an example of a detection signal 1 that
represents the energy of a light beam traveling back along the
laser beam path in Embodiment 2.
[0022] FIG. 11B illustrates an example of a detection signal 2 that
represents the power of a light beam traveling back along the laser
beam path in Embodiment 2.
[0023] FIG. 12 is a flowchart of operation of the laser controller
to detect and address a return beam or self-oscillation beam in
Embodiment 2.
[0024] FIG. 13 is a detailed flowchart of focus adjustment in
Embodiment 2.
[0025] FIG. 14 illustrates a configuration example of an optical
isolator.
[0026] FIG. 15 illustrates a relationship between the temporal
variation in light intensity of the pulse laser beam and the
temporal variation in control voltage applied from a high-voltage
power supply to a Pockels cell.
[0027] FIG. 16A illustrates a configuration example of a beam
adjuster.
[0028] FIG. 16B illustrates a state of the beam adjuster where a
movable plate is moved away from off-axis parabolic convex mirrors
from the state shown in FIG. 16A.
[0029] FIG. 16C illustrates a state of the beam adjuster where a
movable plate is moved closer to the off-axis parabolic convex
mirrors from the state shown in FIG. 16A.
[0030] FIG. 17 is a configuration example of a controller.
DETAILED DESCRIPTION
Contents
[0031] 1. Overview [0032] 2. Overview of EUV Light Generation
System
[0033] Configuration
[0034] Operation [0035] 3. Details of EUV Light Generation
System
[0036] Configuration
[0037] Operation
[0038] Issues [0039] 4. Embodiment 1
[0040] Configuration
[0041] Operation
[0042] Effects [0043] 5. Embodiment 2
[0044] Configuration
[0045] Operation
[0046] Effects [0047] 6. Description of Components [0048] 6.1
Optical Isolator
[0049] Configuration
[0050] Operation [0051] 6.2. Beam Adjuster
[0052] Configuration
[0053] Operation [0054] 6.3 Controllers
[0055] Configuration
[0056] Operation
[0057] Connected Devices
[0058] Hereinafter, selected embodiments of the present disclosure
will be described in detail with reference to the accompanying
drawings. The embodiments to be described below are merely
illustrative in nature and do not limit the scope of the present
disclosure. Further, the configuration(s) and operation(s)
described in each embodiment are not all essential in implementing
the present disclosure. Note that like elements are referenced by
like reference numerals and characters, and duplicate descriptions
thereof will be omitted herein.
1. Overview
[0059] An LPP type EUV light generation system may generate EUV
light by irradiating a target with a pulse laser beam outputted by
a laser apparatus. In order to increase the output power of EUV
light, it may be demanded to increase the power of the laser beam
as well. The laser apparatus may include a master oscillator and an
amplifier.
[0060] To increase the power of the laser beam, a higher-gain
amplifier may be used to raise the amplification rate. However,
this may also increase the intensity of the reflection of the laser
beam off a target (a return beam). Furthermore, the return beam may
travel back along the laser beam path and amplified by the
amplifier to become more intensive. In traveling back along the
laser beam path, the intensive return beam may damage the optical
elements on the laser beam path.
[0061] Meanwhile, in the case where the laser apparatus includes
multiple amplifiers, a self-oscillation beam may be generated. The
self-oscillation beam may travel toward the master oscillator
and/or a target. The self-oscillation beam may be unexpectedly
focused on the laser beam path to damage the optical element at
which the beam is focused.
[0062] An example in this disclosure may analyze the temporal
variation in detection signal of the light traveling back along the
laser beam path to determine whether the detection signal includes
a waveform caused by either a return beam or a self-oscillation
beam. As a result, appropriate measures may be taken against the
generation of a return beam or self-oscillation beam.
2. Overview of EUV Light Generation System
configuration
[0063] FIG. 1 schematically illustrates an exemplary configuration
of an LPP type EUV light generation system. An EUV light generation
apparatus 1 may be used with at least one laser apparatus 3.
Hereinafter, a system that includes the EUV light generation
apparatus 1 and the laser apparatus 3 may be referred to as an EUV
light generation system 11. As shown in FIG. 1 and described in
detail below, the EUV light generation apparatus 1 may include a
chamber 2 and a target supply device 26 (for example, droplet
generator). The chamber 2 may be sealed airtight. The target supply
device 26 may be mounted onto a wall of the chamber 2, for example.
A target material to be supplied by the target supply device 26 may
include, but is not limited to, tin, terbium, gadolinium, lithium,
xenon, or any combination thereof.
[0064] The chamber 2 may have at least one through-hole formed in
its wall. A pulse laser beam 32 outputted from the laser apparatus
3 may travel through the through-hole. At least one window 21 may
be installed on the chamber 2 and the pulse laser beam 32 outputted
from the laser apparatus 3 may travel through the window 21. An EUV
collector mirror 23 having, for example, a spheroidal reflective
surface may be provided in the chamber 2. The EUV collector mirror
23 may have a first focal point and a second focal point.
[0065] The EUV collector mirror 23 may have, for example, a
multi-layered reflective film including alternately laminated
molybdenum layers and silicon layers formed on the surface thereof.
For example, the EUV collector mirror 23 is preferably positioned
such that the first focal point lies in a plasma generation
position (plasma generation region 25) or in the vicinity and the
second focal point lies at a desired focal point defined by the
specifications of an exposure apparatus (an intermediate focal
point 292). The EUV collector mirror 23 may have a through-hole 24
formed at the center thereof and a pulse laser beam 33 may travel
through the through-hole 24.
[0066] The EUV light generation apparatus 1 may include an EUV
light generation controller 5. Further, the EUV light generation
apparatus 1 may include a target sensor 4. The target sensor 4 may
detect at least one of the presence, trajectory, and position of a
target. The target sensor 4 may have an imaging function.
[0067] Further, the EUV light generation apparatus 1 may include a
connection part 29 for allowing the interior of the chamber 2 to be
in communication with the interior of the exposure apparatus 6. A
wall 291 having an aperture may be provided in the connection part
29. The wall 291 may be positioned such that the second focal point
of the EUV collector mirror 23 lies in the aperture.
[0068] The EUV light generation apparatus 1 may also include a
laser beam direction control unit 34, a laser beam focusing mirror
22, and a target collector 28 for collecting targets 27. The laser
beam direction control unit 34 may include an optical element for
defining the travelling direction of the laser beam and an actuator
for adjusting the position or the attitude of the optical
element.
3.2 Operation
[0069] With reference to FIG. 1, a pulse laser beam 31 outputted
from the laser apparatus 3 may pass through the laser beam
direction control unit 34 and, as the pulse laser beam 32, travel
through the window 21 and enter the chamber 2. The pulse laser beam
32 may travel inside the chamber 2 along at least one beam path, be
reflected by the laser beam focusing mirror 22, and strike at least
one target 27 as a pulse laser beam 33.
[0070] The target supply device 26 may be configured to output the
target(s) 27 toward the plasma generation region 25 in the chamber
2. The target 27 may be irradiated with at least one pulse of the
pulse laser beam 33. Upon being irradiated with the pulse laser
beam, the target 27 may be turned into plasma, and EUV light 251
may be generated from the plasma. The EUV light 251 may be
reflected and focused by the EUV collector mirror 23. EUV light 252
reflected by the EUV collector mirror 23 may be outputted through
the intermediate focal point 292 to the exposure apparatus 6. Here,
the target 27 may be irradiated with multiple pulses included in
the pulse laser beam 33.
[0071] The EUV light generation controller 5 may be configured to
integrally control the EUV light generation system 11. The EUV
light generation controller 5 may be configured to process image
data of the target 27 captured by the target sensor 4. The EUV
light generation controller 5 may be configured to control at least
one of: the timing when the target 27 is outputted and the
direction into which the target 27 is outputted, for example. The
EUV light generation controller 5 may be configured to control at
least one of: the timing when the laser apparatus 3 oscillates, the
direction in which the pulse laser beam 32 travels, and the
position at which the pulse laser beam 33 is focused. It will be
appreciated that the various controls mentioned above are merely
examples, and other controls may be added as necessary.
3. Details of EUV Light Generation System
Configuration
[0072] FIG. 2 illustrates details of the configuration example of
the EUV light generation system 11. As shown in FIG. 2, a laser
beam focusing optical system 22a, an EUV collector mirror 23, a
target collector 28, an EUV collector mirror holder 81, plates 82
and 83, a laser beam manipulator 84, and a damper mirror 46 may be
provided within the chamber 2.
[0073] The plate 82 may be anchored to the chamber 2. The EUV
collector mirror 23 may be anchored to the plate 82 with the EUV
collector mirror holder 81. The laser beam focusing optical system
22a may include a convex mirror 221 and a laser beam focusing
mirror 22. The positions and orientations of the convex mirror 221
and the laser beam focusing mirror 22 may be kept to focus the
pulse laser beam 33 reflected by these mirrors at the plasma
generation region 25. The target collector 28 may be disposed upon
a straight line extending from the trajectory of the target 27.
[0074] The laser beam focusing optical system 22a may be provided
on the plate 83. The plate 83 may be connected to the plate 82 with
the laser beam manipulator 84. The plate 83 may be a movable plate.
The laser beam manipulator 84 may be configured to be able to move
the focal point of the pulse laser beam in the directions of the
X-axis, the Y-axis, and the Z-axis to the point specified by the
EUV light generation controller 5 by moving the plate 83 with
respect to the plate 82.
[0075] The damper mirror 46 may be disposed on the laser beam path
at the downstream of the plasma generation region 25 and configured
to reflect the pulse laser beam that has passed through the plasma
generation region 25 toward a beam damper device 47. The damper
mirror 46 may collimate and reflect the incident pulse laser beam
and may be an off-axis parabolic mirror. The damper mirror 46 may
be equipped with a heater for heating its reflective surface to a
temperature equal to or higher than the melting point of the target
material.
[0076] The beam damper device 47 may be installed in the chamber 2.
The beam damper device 47 may be disposed at a place to receive the
pulse laser beam reflected by the damper mirror 46. The beam damper
device 47 may have a damper window provided in the chamber 2
through which the pulse laser beam reflected by the damper mirror
46 will enter.
[0077] The target supply device 26 may be anchored to the chamber
2. The target supply device 26 may hold a target material in a
melted state. A nozzle opening formed in the target supply device
26 may be positioned inside the chamber 2. The target supply device
26 may supply the melted target material to the plasma generation
region 25 within the chamber 2 as droplet-shaped targets 27 through
the nozzle opening. In the present disclosure, the targets 27 may
also be referred to as droplets 27.
[0078] Outside the chamber 2, a laser apparatus 3, a laser beam
direction control unit 34 of a laser beam delivery system, and an
EUV light generation controller 5 may be provided. The laser
apparatus 3 may include a master oscillator (MO) 310, multiple
amplifiers (PA) 311 to 314, an optical isolator 315, a laser
controller 316, and a beam adjuster 317. The laser apparatus 3 may
further include a high-reflectance mirror 318 and not-shown optical
elements on the laser beam path.
[0079] These optical elements may be configured to deliver and/or
shape the laser beam. The master oscillator 310 and the amplifiers
311 to 314 may constitute a master oscillator power amplifier
(MOPA). The amplifiers 311 to 314 may be disposed on the path of
the laser beam outputted from the master oscillator 310.
[0080] The amplifiers 311 to 314 may be connected with PA power
supplies 321 to 324. The PA power supplies 321 to 324 may be
connected with the laser controller 316; the laser controller 316
may be connected with the EUV light generation controller 5.
[0081] The optical isolator 315 may be disposed on the laser beam
path between the master oscillator 310 and the amplifier 311 and
configured to transmit (in an opened state) or block (in a closed
state) the laser beam in accordance with an instruction from the
laser controller 316. The optical isolator 315 will be described
later in detail with reference to FIGS. 14 and 15. The
high-reflectance mirror 318 may be disposed on the laser beam path
between the optical isolator 315 and the amplifier 311.
[0082] The beam adjuster 317 may be disposed on the laser beam path
between the laser beam direction control unit 34 and the laser beam
focusing optical system 22a and configured to adjust the divergence
angle of the laser beam in accordance with an instruction from the
laser controller 316. The beam adjuster 317 will be described later
in detail with reference to FIGS. 16A to 16C.
[0083] The laser beam direction control unit 34 may direct the
pulse laser beam outputted by the laser apparatus 3 to the laser
beam focusing optical system 22a via the window 21. The laser beam
direction control unit 34 may include a high reflectance mirror 342
and a high reflectance mirror 344.
[0084] The EUV light generation controller 5 may receive a control
signal from the exposure apparatus 6. The EUV light generation
controller 5 may control the target supply device 26, the laser
apparatus 3, and the laser beam manipulator 84 in accordance with
the control signal from the exposure apparatus 6.
Operation
[0085] The target supply device 26 may supply droplet-shaped
targets 27 to the plasma generation region 25 at a predetermined
velocity and a predetermined frequency, in accordance with a target
output signal TT from the EUV light generation controller 5. For
example, the target supply device 26 may produce droplets at a
predetermined frequency within a range of several 10 kHz to several
100 kHz.
[0086] A target sensor 4 may detect a target 27 passing through a
specific region. The target sensor 4 may output a passage timing
signal PT as a detection signal of a target 27 to the EUV light
generation controller 5.
[0087] The EUV light generation controller 5 may receive a burst
signal BT from the exposure apparatus 6. The burst signal BT may be
a signal for instructing the EUV light generation system 11 to
generate EUV light within a specified period. The EUV light
generation controller 5 may perform control to output EUV light to
the exposure apparatus 6 during the specified period.
[0088] In the period where the burst signal BT is ON, the EUV light
generation controller 5 may control the laser apparatus 3 to output
a pulse laser beam in accordance with the passage timing signal PT.
In the period where the burst signal BT is OFF, the EUV light
generation controller 5 may control the laser apparatus 3 not to
output a pulse laser beam.
[0089] For example, the EUV light generation controller 5 may
output the burst signal BT received from the exposure apparatus 6
and a light emission trigger signal ET delayed by a predetermined
time from the passage timing signal PT to the laser apparatus 3.
When the burst signal BT is ON, the laser apparatus 3 may output a
pulse laser beam in response to a pulse of the light emission
trigger signal ET.
[0090] The laser controller 316 may output a laser output signal LO
to the master oscillator 310 in response to receipt of the light
emission trigger signal ET. Before this operation, the laser
controller 316 may turn on the PA power supplies 321 to 324. As a
result, the PA power supplies 321 to 324 may supply voltage or
current to the internal electrodes of the corresponding amplifiers
311 to 314 to make the amplifiers 311 to 314 ready for
amplification.
[0091] The master oscillator 310 may output a pulse laser beam
synchronously with the laser output signal LO. The laser controller
316 may open the optical isolator 315 synchronously with the laser
output signal LO. The outputted pulse laser beam may be amplified
by the amplifiers 311 to 314, travel through the laser beam
direction control unit 34, and enter the beam adjuster 317. The
beam adjuster 317 may adjust the divergence angle of the received
pulse laser beam and output the laser beam. The pulse laser beam
outputted from the beam adjuster 317 may travel through the window
21 and enter the chamber 2. The power of the pulse laser beam
outputted from the laser apparatus 3 may reach several kilowatts to
several ten kilowatts.
[0092] The EUV light generation controller 5 may adjust the
outgoing position of the pulse laser beam with the laser beam
manipulator 84. The EUV light generation controller 5 may change
the delay time of the light emission trigger signal ET from the
passage timing signal PT.
[0093] The pulse laser beam may be focused by the laser beam
focusing optical system 22a to hit a target 27 that has arrived at
the plasma generation region 25 and generate EUV light. The
diameter of the pulse laser beam to hit a target 27 may be larger
than the diameter of the target 27; a part of the pulse laser beam
may miss the target 27 and hit the damper mirror 46.
[0094] The pulse laser beam reflected by the damper mirror 46 may
be absorbed by the beam damper device 47 and converted to heat. The
generated heat may be discharged to the external by a not-shown
cooling device.
[0095] The emitted pulse laser beam may not hit a target 27. For
example, the laser apparatus 3 may keep outputting a pulse laser
beam to stabilize the power or to adjust the beam path but
intentionally avoid irradiation by suspending output of a target 27
or changing the delay time. In such a case, the pulse laser beam
may maintain the power and hit the damper mirror 46 without hitting
a target 27.
Issues
[0096] FIG. 3 illustrates the definition of divergence angle. In
FIG. 3, the pulse laser beam 401A traveling in parallel to the
laser beam axis 418 is defined as collimated beam having a
divergence angle .theta.=0. The pulse laser beam 401C traveling in
a direction expanding from the laser beam axis 418 is defined as
pulse laser beam having a positive divergence angle. The pulse
laser beam 401B traveling in a direction converging toward the
laser beam axis 418 is defined as pulse laser beam having a
negative divergence angle. The divergence angle .theta. is defined
by the half angle.
[0097] FIGS. 4A to 4C illustrate states of the return beam
depending on the focused state of the pulse laser beam when hitting
a target 27. The arrowed solid lines in FIGS. 4A to 4C represent a
pulse laser beam to hit a target 27 and the arrowed broken lines
represent a return beam. The pulse laser beam entering the beam
adjuster 317 is assumed to be a collimated beam.
[0098] FIG. 4A illustrates a state of the return beam in the case
where the focal point 413A of the pulse laser beam is set to the
target position. The state where the focal point 413A of the pulse
laser beam is located at the target position is referred to as best
focused state. The pulse laser beam 411A outputted from the beam
adjuster 317 may be a collimated beam. The target 27 before turning
into plasma may reflect the pulse laser beam by surface reflection.
The pulse laser beam reflected off the target 27 may become a
return beam.
[0099] In the case where the focal point of the pulse laser beam is
set to the target position, the return beam may travel back almost
along the optical path of the pulse laser beam to hit a target 27.
The return beam 412A entering the beam adjuster 317 may be a
collimated beam. Furthermore, the return beam may hit and damage
the optical elements defining the upstream optical path of the
pulse laser beam. Particularly in the case where the laser
apparatus 3 is configured as an MOPA including a master oscillator
(MO) 310 and multiple amplifiers (PA) 311 to 314, optical elements
provided in a more upstream region may have lower laser resistance.
In this case, the return beam, which is a part of an amplified
high-power laser beam, may damage the optical elements having low
laser resistance.
[0100] FIG. 4B illustrates a state of the return beam in the case
where the focal point 413B of the pulse laser beam is set to a
position upstream of the target position. The state where the focal
point 413B of the pulse laser beam is located at a position
upstream of the target position is referred to as anteriorly
focused state. The pulse laser beam 411B outputted from the beam
adjuster 317 may be a pulse laser beam having a negative divergence
angle. The target 27 before turning into plasma may reflect the
pulse laser beam by surface reflection. The pulse laser beam
reflected off the target 27 may become a return beam.
[0101] The return beam 412B to enter the beam adjuster 317 may be a
pulse laser beam having a negative divergence angle. The return
beam having a negative divergence angle may be focused in the
middle of the upstream path of the pulse laser beam. For example,
if the return beam is focused in the vicinity of the surface of an
optical element of the laser beam direction control unit 34, the
return beam 412B may damage the optical element. Even if the return
beam 412B is focused in a space other than the surface of an
optical element, the return beam 412B may diverge to heat a
not-shown holder holding an optical element. The thermal
deformation of the heated holder may cause misalignment of the
optical element (in tilt or position).
[0102] In the states of FIGS. 4A and 4B, the return beam may travel
back along the laser beam path from the laser beam focusing optical
system 22a to the master oscillator 310. FIG. 5A schematically
illustrates a temporal waveform of a return beam. The return beam
is a pulse beam and the temporal waveform of the return beam may
have peaks synchronous with laser irradiation of a target.
[0103] FIG. 4C illustrates a state of the return beam in the case
where the focal point 413C of the pulse laser beam is set to a
position downstream of the target position. The state where the
focal point 413C of the pulse laser beam is located at a position
downstream of the target position is referred to as defocused
state. The pulse laser beam 411C outputted from the beam adjuster
317 may be a pulse laser beam having a positive divergence angle.
The target 27 before turning into plasma may reflect the pulse
laser beam by surface reflection. The pulse laser beam reflected
off the target 27 may become a return beam.
[0104] The return beam 412C to enter the beam adjuster 317 may be a
pulse laser beam having a positive divergence angle. The return
beam having a positive divergence angle may expand and travel back
along the optical path of the pulse laser beam; the energy density
thereof may be reduced during the travel. Accordingly, the
possibility that the optical elements defining the upstream laser
beam path could be damaged may be very low.
[0105] As described above, depending on the relation between the
target position and the focal point of the pulse laser beam, the
return beam may cause damage or misalignment of an optical
element.
[0106] Meanwhile, in the case where the laser apparatus 3 includes
multiple amplifiers 311 to 314, a self-oscillation beam may be
generated. The self-oscillation beam is generated when an amplified
spontaneous emission (ASE) beam outputted from an optical amplifier
in an amplifiable state is amplified by another optical amplifier
in an amplifiable state.
[0107] Usually, the laser apparatus 3 may be designed and adjusted
not to generate a self-oscillation beam. However, if an optical
element has damage or a foreign substance on the optical surface,
the reflection off these may be amplified by another amplifier to
become a self-oscillation beam. The self-oscillation beam may be
generated when the alignment of an optical element on the laser
beam path is changed after the initial adjustment.
[0108] A self-oscillation beam may travel toward the master
oscillator and/or the target. The self-oscillation beam may become
an unexpected focused state on the laser beam path and damage the
optical element at the focal point. FIG. 5B schematically
illustrates a temporal waveform of a self-oscillation beam. The
temporal waveform of a self-oscillation beam is an irregular
waveform in which a peak caused by a specific phenomenon cannot be
identified. Compared to the pulse waveform of a return beam, the
waveform of a self-oscillation beam may be a lasting waveform.
[0109] As described above, the self-oscillation beam may be
generated because of damage or misalignment of an optical element.
When a self-oscillation beam is generated, the optical elements may
be damaged more severely. Accordingly, the laser apparatus 3 may
get serious damage. As described above, the damage and misalignment
of an optical element may be caused by a return beam. Accordingly,
the return beam may induce a self-oscillation beam to cause serious
damage of the laser apparatus 3.
[0110] The laser apparatus 3 may be requested to output the pulse
laser beam at higher power. The laser apparatus 3 may be aligned to
the state of FIG. 4C at the initial adjustment. However, as the
heat input to optical elements increases with increase in power of
the pulse laser beam, the optical elements may be thermally
deformed to have thermal lens effect, which may change the
alignment to the state of FIG. 4A or 4B. As a result, a return beam
may be generated. Further, the return beam of the high-power pulse
laser beam has also high power; the optical elements on the laser
beam path may easily become damaged.
[0111] To increase the power of the pulse laser beam, higher-gain
amplifiers may be used to raise the amplification rate. The
high-gain amplifiers may increase ASE to cause self-oscillation
more frequently. For this reason, when the power of the pulse laser
beam is increased, the optical elements may get damaged by a return
beam or self-oscillation beam, so that the laser apparatus 3 may
easily get failed or impaired in performance.
4. Embodiment 1
Configuration
[0112] With reference to FIG. 6, a configuration of an EUV light
generation system 11 in Embodiment 1 is described. The differences
from the configuration in FIG. 2 are mainly described. The laser
apparatus 3 may include a beam splitter 327, a photodetector 328,
and a display device 335.
[0113] The beam splitter 237 may replace the high-reflectance
mirror 318. The beam splitter 327 may be configured to reflect a
part of the incident beam and transmit the remaining of the
incident beam. The photodetector 328 may be disposed to receive the
beam that travels back along the laser beam path and passes through
the beam splitter 327. In the present embodiment, the photodetector
device to detect a beam traveling back along the laser beam path
may be configured with a single photodetector 328.
[0114] The beam traveling back along the laser beam path may
include a return beam and a self-oscillation beam. A not-shown
optical system may be provided between the beam splitter 327 and
the photodetector 328. The optical system may be a collimating
optical system or a focusing optical system. The photodetector 328
may output a temporal waveform of the beam traveling back along the
laser beam path to the laser controller 316. The temporal waveform
of the beam traveling back along the laser beam path may be called
a detection signal.
[0115] The photodetector 328 to receive a beam split by the beam
splitter 327 disposed between the master oscillator 310 and the
amplifier 311 may detect a return beam and a self-oscillation beam
from any of the amplifiers 311 to 314. The beam splitter 327 may be
provided at a different position on the laser beam path.
[0116] The display device 335 may show the information sent from
the laser controller 316 to the operator. The display device 335
may include a monitor and may be a computer including a monitor.
The beam adjuster 317 may be connected with and controlled by the
laser controller 316.
Operation
[0117] The beam splitter 327 may reflect the pulse laser beam
received from the master oscillator 310 via the optical isolator
315 toward the amplifier 311. The beam splitter 327 may transmit
the beam traveling back along the laser beam path and direct the
beam to the photodetector 328. The beam splitter 327 may be
configured to extract a part of the beam traveling back along the
laser beam path and direct it to the photodetector 328.
[0118] The laser controller 316 may receive a detection signal from
the photodetector 328. The laser controller 316 may determine
whether the beam traveling back along the laser beam path includes
a return beam or self-oscillation beam harmful for optical
elements, that is, whether a return beam or self-oscillation beam
is present, based on the detection signal outputted by the
photodetector 328. The laser controller 316 may determine whether a
return beam or self-oscillation beam is present based on the
temporal variation in intensity of the detection signal. The
photodetector 328 may output a detection signal in accordance with
the energy or power of the received light, for example. The
detection signal may be in voltage, for example.
[0119] The laser controller 316 may determine that a
self-oscillation beam is generated when detecting a waveform of the
detection signal in which the time showing values higher than a
threshold lasts long. Furthermore, the laser controller 316 may
determine that a return beam is generated when detecting a waveform
of the detection signal showing pulses at values higher than the
threshold.
[0120] FIG. 7 is a graph for illustrating an example of a way for
the laser controller 316 to determine whether a return beam or
self-oscillation beam is present. FIG. 7 shows two different
detection signals DS_1 and DS_2 of the photodetector 328. The
detection signal DS_1 is an example of a detection signal of a
return beam. The detection signal DS_2 is an example of a detection
signal of a self-oscillation beam. The laser controller 316 may be
provided with a threshold Ic for the intensity of the detection
signal. Furthermore, the laser controller 316 may be provided with
a threshold Tc for the time.
[0121] As to the detection signal DS_1, the signal intensity is
higher than the intensity threshold Ic for the time T_1. As to the
detection signal DS_2, the signal intensity is higher than the
intensity threshold Ic for the time T_2. The time T_1 is shorter
than the time threshold Tc and the time T_2 is longer than the time
threshold Tc.
[0122] The laser controller 316 may measure the time T for which
the intensity of the detection signal is higher than the intensity
threshold Ic and compare the time T with the time threshold Tc to
determine whether a return beam or self-oscillation beam is
present. If the time T is longer than the time threshold Tc, the
laser controller 316 may determine that a self-oscillation beam is
generated. If the time T is shorter than the time threshold Tc, the
laser controller 316 may determine that a return beam is
generated.
[0123] FIG. 8 is a flowchart of operation of the laser controller
316 to detect and address a return beam or self-oscillation beam.
The laser controller 316 may start retrieving the detection signal
outputted by the photodetector 328 (S101).
[0124] The laser controller 316 may determine whether the detection
signal is higher than the intensity threshold Ic (S102). The
intensity threshold Ic may be predetermined based on the threshold
for damage of the optical elements included in the laser apparatus
3. The laser controller 316 may have different intensity thresholds
Ic depending on the operating conditions of the laser apparatus 3
(such as the output energy, the cycle, and the duty). If the
detection signal is higher than the intensity threshold Ic (S102:
Y), the laser controller 316 may start measuring the time T in
which the intensity of the detection signal is higher than the
intensity threshold Ic (S103). If the intensity of the detection
signal is equal to or lower than the intensity threshold Ic (S102:
N), the laser controller 316 may perform S102 again.
[0125] The laser controller 316 may determine whether the time T
has exceeded the time threshold Tc (S104). The laser controller 316
may compare the time T with the time threshold Tc to determine
which of the return beam and the self-oscillation beam is detected.
If the time T has exceeded the time threshold Tc, the laser
controller 316 may determine that a self-oscillation beam has been
detected. If the time T is equal to or shorter than the time
threshold Tc, the laser controller 316 may determine that a return
beam has been detected.
[0126] The laser controller 316 may choose the subsequent
processing depending on which of a return beam and a
self-oscillation beam has been detected. Specifically, in the case
where a self-oscillation beam has been detected (S104: Y), the
laser controller 316 may stop the laser apparatus 3 and then, store
the log of the detection result, provide information to the
operator through the display device 335, and terminate the
processing.
[0127] Specifically, the laser controller 316 may stop the master
oscillator 310, close the optical isolator 315, and further, stop
the PA power supplies 321 to 324 from discharging electricity to
the amplifiers 311 to 314 (S105). The laser controller 316 may
further notify the EUV light generation controller 5 of the stop of
the laser apparatus 3 (S106). The EUV light generation controller 5
may notify the exposure apparatus 6 of the stop of the laser
apparatus 3.
[0128] The laser controller 316 may output a result of detection of
a self-oscillation beam to a log table and store the log table
(S107). The log table may store the operation log of the laser
controller 316 with the time and date. The log table may be stored
in a storage memory 1005, which will be described later with
reference to FIG. 17. The laser controller 316 may store the
information of the stop of the laser apparatus 3 to the log
table.
[0129] The laser controller 316 may output the following
information to the display device 335 (S108): (1) that a
self-oscillation beam has been detected; (2) that the laser
apparatus 3 has been stopped; and (3) that the operator needs to
check the laser apparatus 3 for damage and the laser beam path for
misalignment. The display device 335 may display information on
these matters.
[0130] This display may enable the operator to have a chance to
check the laser apparatus 3 for damage and the laser beam path for
misalignment. The thresholds Ic and Tc may be stored in advance in
the storage memory 1005 of the laser controller 316 by
communication via a network, connection of a storage medium, or
input through a console.
[0131] If a return beam is detected (S104: N), the laser controller
316 may suspend the output of the laser beam, store the log of the
detection result, and provide information to the operator through
the display device 335, and thereafter, adjust the focus.
[0132] Specifically, the laser controller 316 may stop the master
oscillator 310 and close the optical isolator 315 (S109). The laser
controller 316 may keep operating the amplifiers 311 to 314 without
stopping the electricity from the PA power supplies 321 to 324.
This may reduce the time for the later-described focus adjustment
S113. The electricity from the PA power supplies 321 to 324 may be
stopped.
[0133] The laser controller 316 may further notify the EUV light
generation controller 5 of the suspension of the output of the
laser beam (S110). The EUV light generation controller 5 may notify
the exposure apparatus 6 of the suspension of the output of the
laser beam. The laser controller 316 may output a result of
detection of a return beam to the log table and store the log table
(S111). The laser controller 316 may store the information of the
suspension of the output of the laser beam to the log table.
[0134] The laser controller 316 may output the following
information to the display device 335 (S112): (1) that a return
beam has been detected; (2) that the laser output has been
suspended; (3) that focus adjustment will be performed; and (4)
that a laser beam will be intermittently outputted during the focus
adjustment. The display device 335 may display the information on
these matters.
[0135] The laser controller 316 may perform focus adjustment
(S113). The details of the focus adjustment S113 will be described
later with reference to FIG. 9.
[0136] Upon receipt of a laser beam output resumption signal sent
from the EUV light generation controller 5 during or after the
focus adjustment (S114: Y), the laser controller 316 may resume
outputting a laser beam (S115). If the laser controller 316 does
not receive the laser beam output resumption signal (S114: N), the
laser controller 316 may terminate this processing.
[0137] FIG. 9 is a detailed flowchart of the focus adjustment S113.
The laser controller 316 may repeat operation to expand the
divergence angle of the laser beam by a predetermined amount until
the return beam is not detected. The predetermined amount may be a
value determined by experiment.
[0138] Specifically, the laser controller 316 may adjust the beam
adjuster 317 so that the divergence angle of the laser beam
outputted from the beam adjuster 317 increases by a predetermined
amount (S201). Further, the laser controller 316 may make the
master oscillator 310 output a pulse laser beam and open the
optical isolator 315 for a predetermined time (S202). The
predetermined time may be a time sufficiently longer than the time
threshold Tc and determined by experiment.
[0139] The laser controller 316 may receive a detection signal from
the photodetector 328 and determine whether the intensity of the
detection signal is higher than the intensity threshold Ic (S203).
If determining that the intensity of the detection signal is higher
than the intensity threshold Ic (S203: Y), the laser controller 316
may stop the master oscillator 310, close the optical isolator 315
(S204), and return to Step S201.
[0140] If determining at Step S203 that the intensity of the
detection signal is not higher than the intensity threshold Ic
(S203: N), the laser controller 316 may stop the master oscillator
310 and close the optical isolator 315 (S205). Further, the laser
controller 316 may notify the EUV light generation controller 5
that resuming outputting a laser beam is permitted (S206). The EUV
light generation controller 5 may notify the exposure apparatus 6
that resuming outputting a laser beam is permitted.
[0141] The laser controller 316 may output the following
information to the display device 335 (S207): (1) that the return
beam is not detected because of the focus adjustment; (2) that
resuming outputting a laser beam is permitted; and (3) the adjusted
amount in the focus adjustment. The display device 335 may display
information on these matters. The laser controller 316 may store
the result of the focus adjustment to the log table.
Effects
[0142] The present embodiment may correctly determine whether a
beam traveling back along the laser beam path is a return beam or
self-oscillation beam that could damage optical elements, using a
detection signal of one photodetector. Since the present embodiment
may determine whether the beam traveling back along the laser beam
path is light that could damage optical elements and appropriately
address the problem, the damage of the optical elements may be
prevented. In the case of determining that the beam traveling back
along the laser beam path is a return beam, the present embodiment
may perform focus adjustment to reduce the return beam.
Accordingly, even if the thermal lens effect of an optical element
is heightened with increase in power of the laser beam, the damage
of the optical element may be prevented.
[0143] In the case of determining that the beam traveling back
along the laser beam path is a self-oscillation beam, the present
embodiment may immediately stop the laser apparatus 3. Then, the
operator may get a chance to check the optical elements for damage
or misalignment.
[0144] When the gain of the amplifiers is raised with increase in
power of the laser beam, slight damage of an optical element caused
by a return beam may increase the frequency of generation of a
self-oscillation beam. The present embodiment may detect a
self-oscillation beam distinguishingly from a return beam to detect
damage of an optical element at an earlier stage when the power of
the laser beam is increased. The foregoing configuration may
prevent optical elements from being damaged by a return beam or
self-oscillation beam caused by increase in power of the pulse
laser beam outputted from the laser apparatus 3, achieving stable
operation of the laser apparatus 3.
[0145] The laser controller 316 may use different intensity
thresholds and/or different time thresholds for detecting a return
beam and a self-oscillation beam. For example, the intensity
threshold for detecting a self-oscillation beam may be smaller than
the intensity threshold for detecting a return beam. The time
threshold for detecting a self-oscillation beam may be longer than
the time threshold for detecting a return beam.
[0146] The photodetector device for detecting a beam traveling back
along the laser beam path may include photodetectors of the same
type but different in gain. The laser controller 316 may receive
detection signals from the photodetectors of the same type but
different in gain to detect a return beam with the detection signal
of one photodetector and detect a self-oscillation beam with the
detection signal of the other photodetector. The laser apparatus 3
described in the present embodiment may be applicable to a system
different from the EUV light generation system 11. For example, the
laser apparatus 3 may be applicable to a laser annealing apparatus
or a laser processing apparatus. The same applies to Embodiment
2.
5. Embodiment 2
Configuration
[0147] A configuration of an EUV light generation system 11 in
Embodiment 2 is described with reference to FIG. 10. The
differences from the configuration in FIG. 6 are mainly described.
The laser apparatus 3 may include beam splitters 327 and 329, and
photodetectors 331 and 332. The photodetectors 331 and 332 may be
connected with and controlled by the laser controller 316.
[0148] The laser beam manipulator 84 may be connected with and
controlled by the laser controller 316. The laser beam manipulator
84 may be a three-axis stage. The laser beam manipulator 84 may
move the laser beam focusing optical system 22a along the X-axis,
the Y-axis, and the Z-axis in accordance with an instruction from
the laser controller 316.
[0149] The beam splitter 329 may be configured to transmit a part
of the beam transmitted by the beam splitter 327 toward the
photodetector 331 and reflect the remaining part toward the
photodetector 332. The reflectance of the beam splitter 329 may be
larger than 50%. In the present embodiment, the photodetector
device for detecting a beam traveling back along the laser beam
path may be configured with the two photodetectors 331 and 332. The
photodetectors 331 and 332 may receive the light extracted at
different positions on the laser beam path. For example, the light
to be received by the photodetector 332 may be extracted at a
position downstream of the position where the light on the laser
beam path is split for the photodetector 331 to receive.
[0150] The photodetector 331 may be an energy sensor. The
photodetector 331 may output a detection signal 1 representing the
pulse energy of the beam traveling back along the laser beam path
to the laser controller 316. For example, the photodetector 331 may
be a photovoltaic sensor or a mercury cadmium telluride (MCT)
sensor. The photovoltaic sensor is a sensor that makes electrons in
the sensor element absorb optical energy to convert the optical
energy directly to electric energy by photovoltaic effect. The MCT
sensor is a sensor in which the resistance decreases in response to
receiving infrared light.
[0151] The photovoltaic sensor or MCT sensor may output a voltage
or current value representing the pulse energy of the incident
beam. The pulse energy may be expressed as the value (J) obtained
by dividing the peak power by the pulse width. The photovoltaic
sensor or MCT sensor has high responsivity and may be able to
detect a return beam that enters the sensor in the form of a pulse
beam in a relatively short time at each pulse. The response speed
may be several nanoseconds or less.
[0152] The photovoltaic sensor or MCT sensor shows large variation
in sensitivity behavior to variation in temperature of the sensor;
the variation in sensitivity behavior may be 3%/deg or less.
Accordingly, the photovoltaic sensor or MCT sensor may detect a
return beam of a pulse beam more accurately than a self-oscillation
beam that yields relatively large variation in heat input to the
sensor.
[0153] The photodetector 332 may be a power meter. The power meter
may have responsivity lower than the energy sensor. The
photodetector 332 may output a detection signal 2 representing the
power of the beam traveling back along the laser beam path to the
laser controller 316. The photodetector 332 may be a thermopile
sensor. The thermopile sensor is a sensor in which junctions of a
large number of thermocouples connected in series are gathered and
is operated by thermoelectric power.
[0154] The thermopile sensor may output a voltage or current value
representing the power of the incident beam. The power of the
incident beam may be expressed as the average power (W) per unit
time. The thermopile sensor has low responsivity and the response
time may be several ten milliseconds. For this reason, the
thermopile sensor may detect a self-oscillation beam more
accurately than a return beam entering in the form of a pulse beam
in a relatively short time.
[0155] The thermopile sensor shows very small variation in
sensitivity behavior to variation in temperature of the sensor; the
variation in sensitivity behavior may be approximately 0.1%/deg.
Accordingly, the thermopile sensor may detect a self-oscillation
beam that yields relatively large variation in heat input to the
sensor, with relatively high accuracy.
Operation
[0156] The beam splitter 327 may reflect the pulse laser beam
received from the master oscillator 310 via the optical isolator
315 toward the amplifier 311. The beam splitter 327 may transmit
the beam traveling back along the laser beam path and direct the
beam to the beam splitter 329.
[0157] The beam splitter 329 may direct the beam transmitted by the
beam splitter 327 to the photodetectors 331 and 332. If the
reflectance of the beam splitter 327 is configured to be higher
than 50%, the amount of incident light on the photodetector 332 of
a power meter may be larger than the amount of incident light on
the photodetector 331 of an energy sensor. The photodetectors 331
and 332 may respectively output detection signals 1 and 2 to the
laser controller 316.
[0158] FIGS. 11A and 11B illustrate examples of the detection
signals 1 and 2, respectively. The detection signals 1 and 2 may be
voltage signals. The laser controller 316 may determine whether the
beam traveling back along the laser beam path includes a return
beam, which is harmful for the laser apparatus 3, based on the
detection signal 1. Furthermore, the laser controller 316 may
determine whether the beam traveling back along the laser beam path
includes a self-oscillation beam, which is harmful for the laser
apparatus 3, based on the detection signal 2.
[0159] As shown in FIG. 11A, the laser controller 316 may determine
whether the detection signal 1 exceeds a threshold Ec. The laser
controller 316 may determine that a return beam is detected if the
detection signal 1 exceeds the threshold Ec.
[0160] As shown in FIG. 11B, the laser controller 316 may determine
whether the detection signal 2 exceeds a threshold Pc. The laser
controller 316 may determine that a self-oscillation beam is
detected if the detection signal 2 exceeds the threshold Pc.
[0161] FIG. 12 is a flowchart of operation of the laser controller
316 to detect and address a return beam or self-oscillation beam.
The laser controller 316 may start retrieving the detection signals
1 and 2 outputted by the photodetectors 331 and 332 (S301).
[0162] The laser controller 316 may determine whether the intensity
of the detection signal 2 from the photodetector 332 is higher than
the threshold Pc (S302). The threshold Pc may be a voltage
representing a specific power. The threshold Pc may be
predetermined based on the threshold for the damage of the optical
elements included in the laser apparatus 3. The laser controller
316 may have different thresholds Pc depending on the operating
conditions of the laser apparatus 3 (such as the output energy, the
cycle, and the duty).
[0163] If the intensity of the detection signal 2 is higher than
the threshold Pc (S302: Y), the laser controller 316 may determine
that a self-oscillation beam is detected and perform the associated
processing of S303 to S306. Steps S303 to S306 may be the same as
Steps S105 to S108 in the flowchart of FIG. 8 described in
Embodiment 1.
[0164] If the intensity of the detection signal 2 is equal to or
lower than the threshold Pc (S302: N), the laser controller 316 may
determine whether the intensity of the detection signal 1 from the
photodetector 331 is higher than the threshold Ec (S307). The
threshold Ec may be a voltage representing a specific energy. The
threshold Ec may be predetermined based on the threshold for the
damage of the optical elements included in the laser apparatus 3.
The laser controller 316 may have different thresholds Ec depending
on the operating conditions of the laser apparatus 3 (such as the
output energy, the cycle, and the duty).
[0165] If the intensity of the detection signal 1 is higher than
the threshold Ec (S307: Y), the laser controller 316 may determine
that a return beam is detected and perform the associated
processing of S308 to S314. Steps S308 to S314 may be the same as
Steps S109 to S115 in the flowchart of FIG. 8 described in
Embodiment 1.
[0166] The thresholds Ec and Pc may be stored in advance in the
storage memory 1005 of the laser controller 316 by communication
via a network, connection of a storage medium, or input through a
console.
[0167] For the focus adjustment S312, the laser controller 316 may
employ a method different from the focus adjustment S113 in
Embodiment 1. The laser controller 316 may perform the focus
adjustment using the laser beam manipulator 84. FIG. 13 is a
detailed flowchart of the focus adjustment S312 in the present
embodiment.
[0168] The laser controller 316 moves the laser beam focusing
optical system 22a in the +Z direction with the laser beam
manipulator 84 by a predetermined amount (S401). Next, the laser
controller 316 may make the master oscillator 310 output a pulse
laser beam for a predetermined time and open the optical isolator
315 (S402). The predetermined time may be determined by
experiment.
[0169] The laser controller 316 may determine whether the intensity
of the detection signal 1 received from the photodetector 331 is
higher than the threshold Ec (S403). If determining that the
intensity of the detection signal 1 is higher than the threshold Ec
(S403: Y), the laser controller 316 may stop the master oscillator
310 and close the optical isolator 315 (S404), and return to Step
S401.
[0170] If determining at Step S403 that the intensity of the
detection signal 1 is not higher than the threshold Ec (S403: N),
the laser controller 316 may stop the master oscillator 310 and
close the optical isolator 315 (S405). Further, the laser
controller 316 may notify the EUV light generation controller 5
that resuming outputting a laser beam is permitted. The EUV light
generation controller 5 may notify the exposure apparatus 6 that
resuming outputting a laser beam is permitted.
[0171] The laser controller 316 may output the following
information to the display device 335 (S407): (1) that the return
beam is not detected because of the focus adjustment; (2) that
resuming outputting a laser beam is permitted; and (3) the adjusted
amount in the focus adjustment. The display device 335 may display
information on these matters. The laser controller 316 may store
the result of the focus adjustment to the log table.
[0172] Although the focus adjustment S312 has been described as a
different method from the method of the focus adjustment S113 in
Embodiment 1, it may be the same method.
Effects
[0173] The present embodiment may correctly determine whether a
beam traveling back along the laser beam path is a return beam or
self-oscillation beam that could damage optical elements, using
detection signals from two photodetectors different in property.
Since the present embodiment determines whether the beam traveling
back along the laser beam path is light that could damage optical
elements and appropriately address the problem, the damage of the
optical elements may be prevented. If determining that the beam
traveling back along the laser beam path is a return beam, the
present embodiment may perform focus adjustment to reduce the
return beam. Accordingly, even if the power of the laser beam is
raised to increase the thermal lens effect of an optical element,
the damage of the optical element may be prevented.
[0174] If determining that the beam traveling back along the laser
beam path is a self-oscillation beam, the present embodiment may
immediately stop the laser apparatus 3. Then, the operator may get
a chance to check the optical elements for damage or
misalignment.
[0175] When the gain of the amplifiers is raised with increase in
power of the laser beam, slight damage of an optical element caused
by a return beam may increase the frequency of generation of a
self-oscillation beam. The present embodiment may detect a
self-oscillation beam distinguishingly from a return beam to detect
damage of an optical element at an earlier stage when the power of
the laser beam is increased. The foregoing configuration may
prevent optical elements from being damaged by a return beam or
self-oscillation beam caused by increase in power of the pulse
laser beam outputted from the laser apparatus 3, achieving stable
operation of the laser apparatus 3.
6. Description of Components
6.1 Optical Isolator
Configuration
[0176] FIG. 14 illustrates a configuration example of the optical
isolator 315. The optical isolator 315 may include a Pockels cell
501, a first polarizer 502, a second polarizer 503, and a
high-voltage power supply 504. The Pockels cell 501 may be either
an EO Pockels cell or an AO Pockels cell.
[0177] The first polarizer 502 may be disposed on the beam path on
the input side of the Pockels cell 501. The second polarizer 503
may be disposed on the beam path on the output side of the Pockels
cell 501. The high-voltage power supply 504 may output control
voltage for the Pockels cell 501. The high-voltage power supply 504
may receive a control signal from the laser controller 316.
[0178] The high-voltage power supply 504 may generate voltage
predetermined but different from 0 V and apply the voltage to the
Pockels cell 501 when the control signal is ON. The high-voltage
power supply 504 may apply voltage of approximately 0 V to the
Pockels cell 501 when the control signal is OFF.
[0179] FIG. 15 illustrates a relation between the temporal
variation in light intensity of the pulse laser beam and the
temporal variation in control voltage applied from the high-voltage
power supply 504 to the Pockels cell 501. The laser controller 316
may control the high-voltage power supply 504 to apply the
predetermined voltage to the Pockels cell 501 synchronously with
passage of the pulse laser beam outputted from the master
oscillator 310 through the Pockels cell 501.
[0180] The time of applying the predetermined voltage may be longer
than the pulse width of the passing pulse laser beam. For example,
the pulse width may be 20 ns and the time of applying the
predetermined voltage may be 100 ns. The above-described control of
the optical isolator 315 may switch opening and closing the optical
isolator 315 synchronously with the pulse laser beam.
Operation
[0181] The pulse laser beam entering the optical isolator 315 may
travel in the Z direction. The pulse laser beam may be a beam
polarized linearly in the Y direction. The first polarizer 502 may
transmit the pulse laser beam polarized linearly in the Y direction
at high transmittance and reflect light polarized linearly in the X
direction in a direction different from the incident beam path.
[0182] The Pockels cell 501 may rotate the polarization direction
of the pulse laser beam by 90 degrees and transmit the beam when
the predetermined voltage is being applied. The Pockels cell 501
may transmit the pulse laser beam without changing its polarization
direction when voltage at approximately 0 V is being applied.
[0183] The second polarizer 503 may transmit light polarized
linearly in the X direction and reflect light polarized linearly in
the Y direction in a direction different from the path of the pulse
laser beam. That is to say, the second polarizer 503 may transmit
the pulse laser beam of which the polarization direction is rotated
by the Pockels cell 501 when the control voltage is the
predetermined voltage. The second polarizer 503 may reflect the
pulse laser beam of which the polarization direction is not rotated
by the Pockels cell 501 in a direction different from the incident
beam path when the control voltage is approximately 0 V.
[0184] As understood from the above, the optical isolator 315 may
transmit light from the upstream and the downstream when the
predetermined voltage is being applied to the Pockels cell 501 and
block transmission of the light from both of the upstream and the
downstream when the predetermined voltage is not being applied but
the applied voltage is approximately 0 V, functioning as an optical
isolator.
6.2 Beam Adjuster
Configuration
[0185] FIG. 16A illustrates a configuration example of the beam
adjuster 317. The beam adjuster 317 may include two off-axis
parabolic concave mirrors 631 and 634 and two off-axis parabolic
convex mirrors 632 and 633. The off-axis parabolic concave mirror
631, the off-axis parabolic convex mirror 632, the off-axis
parabolic convex mirror 633, and the off-axis parabolic concave
mirror 634 may be disposed on and along the optical path of the
pulse laser beam in this order.
[0186] In the state shown in FIG. 16A, the beam adjuster 317 may be
configured so that the focal point Fl of the off-axis parabolic
concave mirror 631 is located at the same position as the focal
point F2 of the off-axis parabolic convex mirror 632. Furthermore,
the beam adjuster 317 may be configured so that the focal point F3
of the off-axis parabolic convex mirror 633 is located at the same
position as the focal point F4 of the off-axis parabolic concave
mirror 634.
[0187] The off-axis parabolic concave mirrors 631, 634 and the
off-axis parabolic convex mirrors 632, 633 may be disposed so that
the optical axis OA2 between the off-axis parabolic concave mirror
631 and the off-axis parabolic convex mirror 632 is parallel to the
optical axis OA4 between the off-axis parabolic convex mirror 633
and the off-axis parabolic concave mirror 634.
[0188] The off-axis parabolic concave mirrors 631, 634 and the
off-axis parabolic convex mirrors 632, 633 may be disposed so that
the optical axis OA1 of the pulse laser beam incident on the
off-axis parabolic concave mirror 631 matches the optical axis OA5
of the pulse laser beam outgoing from the off-axis parabolic
concave mirror 634.
[0189] The off-axis parabolic concave mirrors 631, 634 and the
off-axis parabolic convex mirrors 632, 633 may be disposed so that
the optical axis OA3 between the off-axis parabolic convex mirror
632 and the off-axis parabolic convex mirror 633 is parallel to the
optical axis OA1 of the pulse laser beam incident on the off-axis
parabolic concave mirror 631 and the optical axis OA5 of the pulse
laser beam outgoing from the off-axis parabolic concave mirror
634.
[0190] The distance between the off-axis parabolic convex mirror
632 and the off-axis parabolic concave mirror 631 may be equal to
the distance between the off-axis parabolic convex mirror 633 and
the off-axis parabolic concave mirror 634. These distances are
denoted by H.
[0191] The beam adjuster 317 may further include a base plate 638
and a one-axially movable stage 635. The one-axially movable stage
635 may include a movable plate 637 movable on the one-axially
movable stage 635 in a one-axis direction.
[0192] The one-axially movable stage 635 may be disposed on the
base plate 638 and configured to be able to move the movable plate
637 with respect to the base plate 638. The direction of moving the
movable plate 637 may be parallel to the optical axis OA2 and the
optical axis OA4. The off-axis parabolic concave mirrors 631 and
634 may be anchored to the base plate 638. The off-axis parabolic
convex mirrors 632 and 633 may be anchored to the movable plate
637.
Operation
[0193] The operation of the beam adjuster 317 is described with
reference to FIGS. 16A to 16C. FIG. 16B shows a state of the beam
adjuster 317 where the movable plate 637 is moved away from the
off-axis parabolic concave mirrors 631 and 634 from the state shown
in FIG. 16A. FIG. 16C shows a state of the beam adjuster 317 where
the movable plate 637 is moved closer to the off-axis parabolic
concave mirrors 631 and 634 from the state shown in FIG. 16A.
[0194] In FIG. 16A, the pulse laser beam to hit the off-axis
parabolic concave mirror 631 may be a collimated beam. The off-axis
parabolic concave mirror 631 may reflect the pulse laser beam to
focus the pulse laser beam at the focal point F1. The off-axis
parabolic convex mirror 632 may convert the pulse laser beam
reflected by the off-axis parabolic concave mirror 631 and
traveling to be focused at the focal point F1 into a collimated
beam and reflect it. The beam diameter BD2 of the pulse laser beam
converted by the off-axis parabolic convex mirror 632 into a
collimated beam may be reduced to 1/M12 of the incident beam
diameter BD1.
[0195] The focal length of the off-axis parabolic concave mirror
631 is defined as LF1 and the focal length of the off-axis
parabolic convex mirror 632 is defined as LF2. As described above,
the focal point F1 may be located at the same position as the focal
point F2. The magnification M12 may be LF1/LF2. The distance H
between the off-axis parabolic concave mirror 631 and the off-axis
parabolic convex mirror 632 may be LF1-LF2.
[0196] The pulse laser beam converted into a collimated beam having
a beam diameter BD2 may be reflected by the off-axis parabolic
convex mirror 633 like a pulse laser beam that diverges from the
focal point F3. As described above, the focal point F3 may be
located at the same position as the focal point F4. Accordingly,
the off-axis parabolic concave mirror 634 may convert the pulse
laser beam diverging from the focal point F3 into a collimated beam
having an optical axis OA5, which is substantially the same as the
optical axis of the pulse laser beam incident on the off-axis
parabolic concave mirror 631, and reflect it.
[0197] The beam diameter of the pulse laser beam reflected by the
off-axis parabolic convex mirror 633 to hit the off-axis parabolic
concave mirror 634 may be enlarged by a magnification M43. The
focal length of the off-axis parabolic convex mirror 633 is defined
as LF3 and the focal length of the off-axis parabolic concave
mirror 634 is defined as LF4. The magnification M43 may be LF4/LF3.
If LF1=LF4 and LF2=LF3, the magnification M12 may be equal to the
magnification M43. Accordingly, the beam diameter BD3 of the
outgoing light from the off-axis parabolic concave mirror 634 may
be the same as the beam diameter BD1 of the pulse laser beam
incident on the off-axis parabolic concave mirror 631.
[0198] The one-axially movable stage 635 may move the movable plate
637 with respect to the base plate 638 in accordance with the
control of the laser controller 316. The laser controller 316 may
increase or decrease the distance H between the off-axis parabolic
concave mirror 631 and the off-axis parabolic convex mirror 632 by
moving the movable plate 637. By changing the distance H, the laser
controller 316 may collect or diverge the outgoing beam from the
beam adjuster 317.
[0199] As shown in FIG. 16B, the laser controller 316 may increase
the distance H from the state of FIG. 3A by dL. The divergence
angle of the pulse laser beam outgoing from the off-axis parabolic
concave mirror 634 may decrease. The beam diameter BD3 of the pulse
laser beam outgoing from the off-axis parabolic concave mirror 634
may become slightly smaller than but substantially equal to the
beam diameter BD1 of the pulse laser beam incident on the off-axis
parabolic concave mirror 631. Furthermore, the optical axis OA1 of
the pulse laser beam incident on the off-axis parabolic concave
mirror 631 may match the optical axis OA5 of the pulse laser beam
outgoing from the off-axis parabolic concave mirror 634.
[0200] As shown in FIG. 16C, the laser controller 316 may decrease
the distance H from the state of FIG. 16A by dL. The divergence
angle of the pulse laser beam outgoing from the off-axis parabolic
concave mirror 634 may increase. The beam diameter BD3 of the pulse
laser beam outgoing from the off-axis parabolic concave mirror 634
may become slightly larger than but substantially equal to the beam
diameter BD1 of the pulse laser beam incident on the off-axis
parabolic concave mirror 631. Furthermore, the optical axis OA1 of
the pulse laser beam incident on the off-axis parabolic concave
mirror 631 may match the optical axis OA5 of the pulse laser beam
outgoing from the off-axis parabolic concave mirror 634.
6.3 Controllers
[0201] The aforementioned controllers in the present disclosure,
inclusive of the laser controller 316 and the EUV light generation
controller 5, may be configured with a general-use control
apparatus, such as a computer or a programmable controller. For
example, the controllers may have the following configuration.
Configuration
[0202] FIG. 17 illustrates a configuration of a controller. The
controller may include a processing unit 1000 and further, a
storage memory 1005, a user interface 1010, a parallel I/O
controller 1020, a serial I/O controller 1030, an A/D and D/A
converter 1040 connected with the processing unit 1000. The
processing unit 1000 may include a CPU 1001, and a memory 1002, a
timer 1003, and a GPU 1004 connected with the CPU 1001.
Operation
[0203] The processing unit 1000 may retrieve a program stored in
the storage memory 1005. The processing unit 1000 may execute the
retrieved program, retrieve data from the storage memory 1005 in
accordance with the executed program, and store data to the storage
memory 1005.
[0204] The parallel I/O controller 1020 may be connected with a
device that is capable of communicating via a parallel I/O port.
The parallel I/O controller 1020 may control digital signal
communication via the parallel I/O port performed by the processing
unit 1000 executing a program.
[0205] The serial I/O controller 1030 may be connected with a
device that is capable of communicating via a serial I/O port. The
serial I/O controller 1030 may control digital signal communication
via the serial I/O port performed by the processing unit 1000
executing a program.
[0206] The A/D and D/A converter 1040 may be connected with a
device that is capable of communicating via an analog port. The A/D
and D/A converter 1040 may control analog signal communication via
the analog port performed by the processing unit 1000 executing a
program.
[0207] The user interface 1010 may be configured for the operator
to instruct the processing unit 1000 to display the process of
executing a program, or abort or interrupt the execution of a
program.
[0208] The CPU 1001 of the processing unit 1000 may perform
arithmetic operation of a program. The memory 1002 may temporarily
store a program or data generated in the arithmetic operation when
the CPU 1001 executes the program. The timer 1003 may measure the
time point or a passage time and output the time point or the
passage time to the CPU 1001 in accordance with the executed
program. The GPU 1004 may process image data received by the
processing unit 1000 in accordance with the executed program and
output the result to the CPU 1001.
Connected Devices
[0209] The devices that are connected with the parallel I/O
controller 1020 and capable of communicating via a parallel I/O
port may include the display device 335 and the other controllers.
The devices that are connected with the serial I/O controller 1030
and capable of communicating via a serial I/O port may include the
master oscillator 310, the PA power supplies 321 to 324, the
optical isolator 315, the beam adjuster 317, and the laser beam
manipulator 84. The devices that are connected with the A/D and D/A
converter 1040 and capable of communicating via an analog port may
include the photodetectors 328, 331, and 332.
[0210] The foregoing description is merely for the purpose of
exemplification but not limitation. Accordingly, it is obvious for
a person skilled in the art that the embodiments in this disclosure
can be modified within the scope of the appended claims.
[0211] A part of the configuration of an embodiment can be replaced
with a configuration of another embodiment. A configuration of an
embodiment can be incorporated to a configuration of another
embodiment. A part of the configuration of each embodiment can be
removed, added to a different configuration, or replaced by a
different configuration.
[0212] The terms used in this specification and the appended claims
should be interpreted as "non-limiting". For example, the terms
"include" and "be included" should be interpreted as "including the
stated elements but not limited to the stated elements". The term
"have" should be interpreted as "having the stated elements but not
limited to the stated elements". Further, the modifier "one (a/an)"
should be interpreted as "at least one" or "one or more."
* * * * *